Airline Communications and Reporting System (ACARS) is a character-oriented radio frequency (RF) datalink system currently in use by airlines worldwide for a variety of Air Traffic Services (ATS), Airline Administrative Control (AAC), Aeronautical Operational Control (AOC), and Airline Passenger Communications (APC) datalink applications. ACARS employs a modulation scheme and media control protocol over open RF channels that achieve a raw throughput of 2,400 bits per second. As defined in the Aeronautical Radio, INCorporated (ARINC) 618-6 Air/Ground Character-Oriented Protocol Specification associated with ACARS, a set of American Standard Code for Information Interchange (ASCII) encoded characters may be transmitted over the system. As depicted in FIG. 1, transmission of data from an aircraft 10 to a ground network 12 typically involves an on-board Communications Management Unit (CMU) that automatically selects among transmission media including Very High Frequency (VHF) 14, High Frequency (HF) 16 and SATellite COMmunications (SATCOM) systems that include a satellite 18 and a satellite ground station 19. A CMU can alternatively be a function within a larger integrated system such as a combined Flight Management System that contains a CMU function or an Integrated Modular Avionics System with an included CMU function.
VHF Datalink Mode 2 (VDL-2) is a bit-oriented datalink system that provides high-speed, datalink functions over the same aviation VHF frequency spectrum currently supporting ACARS. VDL-2 achieves a raw data rate of 31,500 bits per second (i.e. a 13 times increase in throughput over the ACARS datalink). Via its datalink protocol, referred to as Aviation VHF Link Control (AVLC), VDL-2 provides for fully binary messages as opposed to the character-oriented ACARS.
Because VDL-2 operates in the same spectrum as ACARS at a much higher throughput, many experts in the airline industry view VDL-2 as the successor to ACARS. Even as the airline industry converts the existing ACARS systems into the faster VDL-2, VHF ground stations must support both ACARS and VDL-2 datalinks. During and after the transition, VDL-2 will include support for “legacy” ACARS applications for an extended period of time. To this end, a protocol referred to as ACARS over AVLC (AOA) defines a method of message encapsulation that enables the operation of ACARS applications over the VDL-2 datalink. The AOA protocol defines an AOA packet that encapsulates an ACARS message, which is further encapsulated into an AVLC frame. The AOA protocol differentiates AVLC frames containing AOA packets from other AVLC frames (such as AVLC containing ATN packets) with a 2-byte header where the first byte is known as the Initial Protocol Identifier (IPI) and the second byte is known as the Extended Protocol Identifier (EPI). The IPI and EPI may each be set with a preconfigured value to identify an encapsulated ACARS message. In operation, a Data Link Processor (DLP) at a VDL-2 AVLC interface identifies and directs encapsulated ACARS frames to the proper end-system.
In order to support modern bit-oriented applications over ACARS, ARINC developed a protocol (i.e. ARINC 622) that causes each 4-bit block of a binary data stream to be converted into a hexadecimal ACARS character. While the ARINC 622 protocol does provide a transition mechanism from existing ACARS systems to more modern binary aviation VHF datalinks, the ARINC 622 protocol results in a doubling of the message size because each 4 bits of binary data is encoded and transmitted as a one 8-bit ACARS character.